In many pump applications utilizing a filter, it is recognized that the filter may eventually be clogged by substances in the material being pumped. This can be a critical problem in fuel pumps for aircraft, for example, where there may be a failure to provide fuel to an engine under such circumstances. Because of the critical nature of such fuel pumps, it has been necessary to devise bypass valves for diverting fuel around a clogged filter during flight.
Since the pressure upstream of the filter in the outlet of the pump will rise in the event the filter is clogged, it is desirable to provide a pressure sensitive bypass valve in the outlet to divert the fuel around the filter at a selected pressure above the normal operating pressure. This will ensure continuing operation of the aircraft since the engine will receive an uninterrupted flow of fuel through the bypass valve. However, while it is acceptable to operate the aircraft on unfiltered fuel for a relatively short time period, the filter should be replaced in accordance with normal maintenance procedures before any prolonged aircraft operation.
Because of this important factor in aircraft maintenance, the pilot should be given a warning at the instant the aircraft is operating on unfiltered fuel. In fact, as an extra precaution, the pilot should ideally receive advance notice of a pressure build up in the outlet upstream of the filter even prior to the time the bypass valve opens to divert fuel around the filter, after which the filter may be replaced utilizing normal maintenance procedures even before the filter becomes clogged to the point of causing the bypass valve to be opened. Despite the recognized desirability for an integral switch and bypass valve, there has been no known commercially acceptable fuel pump incorporating components in a reliable fashion.
Among the various fuel pumps proposed is that disclosed in Prigatel U.S. Pat. No. 3,182,596. This fuel pump is of the intermeshing gear type and is principally directed to overcoming the problems presented when bubbles are present in the fluid entering the pump inlet. For this purpose, the pump includes a pressure relief valve permitting fuel in auxiliary inlet passages to return to a main inlet passage.
Similarly, Henry U.S. Pat. No. 3,146,720 discloses a pressure relief valve for a hydraulic pump of the positive displacement type. The pressure relief valve is utilized with a gear pump to control the effects of variations or pulsations in the outlet in response to the quantities or surges of fluid positively displaced by the gear pump. However, the pressure relief means neither bypasses a filter nor actuates an indicator.
A fuel burner regulating valve assembly is disclosed in commonly owned Swedberg U.S. Pat. No. 3,566,901. This fuel regulating valve includes a main regulating valve for controlling the supply of fuel to a burner nozzle and a flow responsive valve upstream of the main regulating valve for bypassing fluid from the main valve until a predetermined flow rate is achieved, together with a variable orifice for providing a pressure drop that controls the flow responsive valve. With the variable orifice located in the supply passage to the main regulating valve, it is automatically operative to change its effective area with flow rates above a predetermined flow rate to maintain a substantially constant pressure drop thereacross.
Despite the efforts to control or eliminate the problems that are encountered in utilizing fuel pumps, there has been no integral switch and bypass valve for such pumps. It is, therefore, desirable to provide such an arrangment which operates in a highly effective and reliable manner to permit fuel to bypass a clogged filter in order to ensure uninterrupted flow of fuel to an engine during flight. Accordingly, the present invention is directed to overcoming the above stated problems and accomplishing the stated objects.